UTRAN, short for UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network, is a collective term for the Base Stations and Radio Network Controllers (RNCs) which make up the UMTS radio access network. This communications network, commonly referred to as 3G (for 3rd Generation Wireless Mobile Communication Technology), can carry many traffic types from real-time Circuit Switched to IP based Packet Switched. The UTRAN allows connectivity between the User Equipment (UE) and the Core Network (CN). The UTRAN includes the base stations, which are called Node Bs, and Radio Network Controllers (RNCs). The RNC provides control functionalities for one or more Node Bs. A Node B and an RNC can be the same device (otherwise known as eHSPA NodeB), although typical implementations have a separate RNC located in a central office serving multiple Node B's. Despite the fact that they do not have to be physically separated, there is a logical interface between them known as the Iub. The RNC and its corresponding Node Bs are called the Radio Network Subsystem (RNS). There can be more than one RNS present in an UTRAN.
In the UTRAN architecture, as depicted in FIG. 1, there are four interfaces connecting the UTRAN to other functional entities: Iu, Uu, Iub and Iur. The Iu interface is an interface that connects the RNC to the CN. The Uu interface connected the Node B with the UE. The Iub is an interface connects the RNC with the Node B. The Iur connects two RNCs with each other. It is understood that the functional entities, such as the RNC, Node B, etc, may be any device capable of processing the necessary functions, as readily understood by the skilled artisan.
UMTS is one of the third-generation (3G) cell phone technologies. Currently, the most common form uses W-CDMA as the underlying air interface, is standardized by the 3GPP, and is the European answer to the ITU IMT-2000 requirements for 3G cellular radio systems. UMTS, using W-CDMA, supports up to 14.0 Mbit/s data transfer rates in theory (with HSDPA), although at the moment users in deployed networks can expect a transfer rate of up to 384 kbit/s for R99 handsets, and 3.6 Mbit/s for HSDPA handsets in the downlink connection. This is still much greater than the 9.6 kbit/s of a single GSM error-corrected circuit switched data channel or multiple 9.6 kbit/s channels in HSCSD (14.4 kbit/s for CDMAOne), and—in competition to other network technologies such as CDMA2000, PHS or WLAN—offers access to the World Wide Web and other data services on mobile devices. Of course, 3GPP is continually improving HSPA technology, with MIMO and 64QAM, such that the peak rate is much larger.
Since 2006, UMTS networks in many countries have been or are in the process of being upgraded with High Speed Downlink Packet Access (HSDPA), sometimes known as 3.5G. Currently, HSDPA enables downlink transfer speeds of up to 7.2 Mbit/s. Work is also progressing on improving the uplink transfer speed with the High-Speed Uplink Packet Access (HSUPA).
In mobile telecommunication systems, the ability to allow subscribers to move within and outside networks is important. This requires a process known as Serving RNS (SRNS) relocation, in which an existing SRNS is replaced by a target RNS. When a UE moves out of a first RNS into a new RNS, a process known as Hard Handover (HHO), the service RNC (SRNC) of the SRNS is replaced by the target RNC of the second RNS. For example, the user place connection extends between the target RNC (i.e. drift RNC) and the CN via the SRNC, where the interface between the two is the Iur interface. The network may subsequently divide to convert the drift RNC (DRNC) into the SRNC, establishing a direct user plane connection to the CN.
The SRNS relocation process may be initiated, for example, by the SRNC sending to the CN a Relocation Required message. This message is defined in the Radio Access Network Application Part (RANAP) protocol and carried over the Iu interface. The CN responds to receipt of a Relocation Required message by sending a Relocation Request message to the target RNC. The Relocation Request message is also defined in the RANAP protocol.
The Relocation Request message contains the identities of the Radio Access Bearers (RABs) to be transferred. Both the Relocation Request message and the Relocation Required message contain an Radio Resource Control (RRC) container. This container is defined in the RRC protocol, and contains amongst other things the identities of the Radio Bearers (RBs) to be transferred as well as details of the mappings between the RBs and the lower layer attributes, i.e. to logical, transport, and physical channels. It is noted that the RB identities and the mappings between the RBs and the lower layer attributes are contained in a container which is passed transparently by the core network.
FIG. 2 illustrates an alternative architecture depicting an evolved HSPA architecture for a flat radio access architecture of an UTRAN network. In this architecture, the RNC functions are in Node B, and referred to as an evolved HSPA Node B. This architecture does not have an Iu-CS UP (interface toward CS-Service Domain) with the Mobile Switching Center (MSC) or with Media Gateway (MGW), and in the cell which both CS and PS service is operated under the Node B and when CS service is to be established (UE requests a CS RAB), the Node B+ needs to execute the SRNS Relocation for moving the Serving RNC's (SRNCs) functionality to the RNC which has an Iu-CS UP.